1. Field of the Invention
The present invention relates to a comparator for comparing an input signal with a reference signal, and an analog-to-digital converter using the comparator.
2. Description of the Related Art
In general, a comparator (analog comparator) is configured to compare an analog input voltage with a reference voltage, and output a digital signal of a high (H) level or a low (L) level according to a comparison result. Further, in the analog-to-digital converter (hereinafter simply referred to as an ADC), a plurality of comparators having different reference voltages are used to convert an analog input voltage into a digital signal. In an ordinary ADC, a voltage generating circuit for generating a plurality of different reference voltages becomes necessary.
As a method of configuring a comparator, a comparator including a circuit in which input sections and output sections of two inverters are connected to each other in an annular form is known. According to “Thomas Bynghak Cho, and Paul R. Gray, “Thomas Byunghak Cho, and Paul R. Gray, “A 10 b, 20 Msample/s 35 mW Pipeline A/D Converter”, IEEE J. of Solid-State Circuits Vol. 30, No. 3, March 1995. (FIG. 1)” (related art), as one of comparators having the annular circuit, a comparator of a built-in threshold type is described.
In the built-in threshold type comparator, a variable resistor a resistance value of which changes according to the input voltage and the reference voltage is provided between each of both the inverters and the power source (high potential power source) or between each of both the inverters and the ground power source (low potential power source). Each of both the inverters is provided with two switches (transistor switches) which are called a reset switch, and an on/off switch. The reset switch is used to reset an output voltage of the inverter to the power source voltage or the ground power source voltage. The on/off switch is inserted in series between an NMOS transistor and a PMOS transistor of the inverter, and is used to control turning on/off of the inverter by closing or opening the connection between the two transistors. A description will be given below on the assumption that the variable resistor is provided between each of both the inverters and the ground power source, and the reset switch is used to reset the output voltage of the inverter to the power source voltage.
In the built-in threshold type comparator, the output is reset before the comparison operation is started. That is, the reset switches of both the inverters are set ON, and the outputs of both the inverters coincide with the power source voltage. As a result of this, it is possible to prevent an influence of the previous comparison result from propagating to the next comparison result. Further, during the period in which the reset switch is in the on-state, the on/off switch is set in the off-state, and thus the shoot-through current is suppressed, thereby reducing the power consumption.
After the above-mentioned resetting is completed, the comparison operation is performed. At the time of the comparison operation, the reset switch is set in the off-state, and the on/off switch is set in the on-state. Then, a current flows from an output terminal of each of both the inverters to the variable resistor, and a voltage drop is caused. At this time, the resistance values of both the variable resistors differ from each other depending on the values of the input voltage and the reference voltage, and hence the voltage falling speeds of both the inverter outputs differ from each other. As a result of this, a latch state where the one inverter output is high, and the other inverter output is low is obtained.
In the ADC using such a built-in threshold type comparator, the reference voltage can be scaled up/down by changing the size ratio of the transistor for receiving the input voltage to the transistor for receiving the reference voltage. Accordingly, the number of reference voltages to be generated may be one, and a voltage generating circuit for generating a plurality of reference voltages is not necessary unlike the ordinary ADC.
In the built-in threshold type comparator described in the related art, the operation speed is lowered by the ON resistance of the on/off switch. If the size of the transistor used for the on/off switch is made larger, the ON resistance can be made small. In this case, however, a capacitance of the parasitic capacitor is increased concomitantly with an increase in size. If the parasitic capacity is increased, the operation speed of the comparator is lowered, and hence increasing the transistor size of the on/off switch is not always connected with improvement in the operation speed.